The present invention relates to a flat-plate lens suitable for use within an optical path from a light source to the recording layer or from the recording layer to an optical detector for conducting recording of information or retrieving of the recorded information by utilizing optical means. The flat-plate lens exhibits uniformly imaging performance at any position on the lens body so as to provide a high-density optical recording method and a high-density optical recording medium by utilizing the method.
In recent years, optical recording media are highlighted as a recording medium to play an indispensable role among various recording media in the information-predominant society by virtue of their excellent performance for recording and retrieving information in a high density at a high velocity. Several types of optical recording media are already under practical applications with a high recording density including DVD-RAM, DVD-R, DVD-RW and others.
In the optical recording of information, the density of information recording and retrieving is limited by the critical diffraction determined depending on the wavelength of the light used. As a technology for overcoming this problem of limitation, an optical recording method by utilizing near-field light has recently come to be highlighted. Existence of the near-field light, however, is limited to the very proximity apart by several tens of nanometers from the generation source of the near-field light with rapidly decreasing intensity as the distance from the source is increased. This matter is the largest limiting factor in the technology of optical recording by utilizing the near-field light.
A proposal is made for a so-called Super-RENS (Super Resolution Near-field Structure) with a near-field light source built in the optical recording medium enabling high-density information recording with recording marks having a diameter as small as 60 nm or even smaller. See, for example, Japanese Patent Kokai 11-250493 and Applied Physics Letters, volume 73, page 2078 (1998). A problem in this Super-RENS is that the information recording layer is adversely affected by the heat generated in the near-field light generating source even though the recording layer is isolated from the near-field light source with intervention of a protecting layer.
As a solution for this problem, a so-called perfect lens is proposed by using a material having a refractive index of −1 as a means for obtaining an image of the near-field light source. See, for example, Physical Review Letters, volume 85, page 3966 (2000). It is, however, practically a difficult matter to conduct optical recording by using such a very special material having a refractive index of −1 or a negative value.
Besides, a study is now under the way for the solution of the above problem in which a hologram recording is made of the near-field light source and the reconstructed image thereof is utilized for optical information recording. See Optics Letters, volume 26, page 1800 (2001). There would still be a very long way, however, before the study has reached a stage for practical application of the method in an actual optical recording system.
As described above, it is the present status of the optical information recording technology that no lens systems are available for obtaining optical imaging in a diameter of the image not exceeding 100 nm.